Technical Field
The present invention relates to flow computation, and more particularly to systems and methods for estimating fractional flow reserve from angiograms.
Description of the Related Art
In the treatment of coronary artery disease, there are a number of tools available to a clinician, including a traditional x-ray angiogram, as well as more advanced fractional flow reserve (FFR) or optical coherence tomography (OCT) techniques. With an x-ray angiogram, a catheter is inserted into a major systemic artery and guided to the coronary vasculature where a radio-opaque dye is released that will mix with the flowing blood and reveal the vascular geometry. While this technique can provide anatomical information regarding stenosed lesions, the well-known ‘FAME’ trial indicated that patient outcomes were significantly improved when treatment was guided by a combination of both anatomical and functional information, as provided by FFR.
With this latter technique, a different catheter equipped with a pressure transducer is inserted and guided to a stenosed region of the coronary vasculature and transient pressure recordings are acquired both upstream and downstream of the lesion such that a measure of the pressure drop can be made. Despite the effectiveness of FFR, the increased cost of both a disposable catheter and clinician's time, plus the increased radiation exposure and potential damage to the arterial wall has meant that it is only utilized in a relatively small proportion of cases.
For this reason, there is a desire to quantify the pressure drop through a stenosis by other means. One promising technique is computational fluid dynamics. In this scenario, a computer model of the coronary vasculature can be created from an x-ray angiogram using either commercial or in-house software, then subsequently a three-dimensional (3D) computational mesh is created and the incompressible Navier-Stokes equations may be solved to obtain the velocity and pressure fields, known as virtual FFR or vFFR. Shortcomings of these techniques are in terms of the workflow and lead time necessary to produce a result and/or the specification of boundary conditions necessary to obtain a unique and patient-specific solution to the Navier-Stokes equations.
The workflow for extracting a geometrical model of a coronary vasculature, namely creating a 3D computational mesh, running a computational fluid dynamics (CFD) simulation, and post-processing the result, tends to be performed with a series of commercial engineering (or similar style) software packages, which would require input from a user skilled in this field and may be infeasible in a clinical setting.
Failing that, a medical image dataset may be uploaded through a web browser with the workflow performed elsewhere, but in either case the time required will be far greater than that for an FFR procedure. In terms of obtaining a unique and patient-specific model of the hemodynamic flow field in the coronary arteries, the options are either to specify pressures at the inlet and outlet/s of the model, or specify velocities (or equivalently flow rates) at the inlet and some form of ‘outflow’ condition at the outlets.
In practice, none of this information is readily available since flow rates are not measured as part of a standard angiographic procedure and while pressures could be recorded, this would require performing an FFR procedure, which would circumvent the need for performing the CFD simulation in the first place. While it may be tempting to try and use ‘standard’ values for flow rates or pressures in the larger arterial branches, this is problematic given the very patient-specific form of a stenosis, combined with the fact that vasodilatory drugs are given to increase the flow rate and hence pressure drop through the artery while pressure measurements are made.
In recent years, there have been significant efforts in the application of CFD to compute FFR. A vFFR measurement approach may be based on using computed tomography (CT) data to generate the geometry, which is acquired in a different clinical setting to where an x-ray angiogram is employed. As such, the turnaround time is much longer than would be acceptable in an interventional cardiology procedure. Furthermore, this service does not address the issue of patient-specific boundary conditions to the Navier-Stokes equations, which are needed in obtaining accurate results.